<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <input type="text">
    <select name="" id="">
        <option value="" selected>+</option>
        <option value="">-</option>
        <option value="">*</option>
        <option value="">/</option>
    </select>
    <input type="text">
    <button type="button">=</button>
    <input type="text">
</body>
<script src="js/jquery-1.12.4.min.js"></script>
<script>
    const $inputs = $("input");
    $("button").click(function () {
      const a1 = parseInt($inputs.eq(0).val());
      const a2 = parseInt($inputs.eq(1).val());
      const fuhao = $("option:selected").text();
      if (fuhao === "+") {
        $inputs.eq(2).val(a1+a2);
      } else if (fuhao === "-") {
        $inputs.eq(2).val(a1-a2);
      } else if (fuhao === "*") {
        $inputs.eq(2).val(a1*a2);
      } else if (fuhao === "/") {
        $inputs.eq(2).val(a1/a2);
      }
    })
</script>
</html>